This invention pertains to control rods for nuclear reactors and more particularly to a coupling indicator for control rod assemblies having remotely disengageable couplings between the control rod and the control rod drive shaft.
A nuclear power plant generates electricity from heat produced by fissioning of a fissile material. The fissile material, or nuclear fuel, is contained within a fuel assembly; a plurality of fuel assemblies comprising a nuclear core. In order to extract the nuclear heat produced by fissioning of the fuel, the core is placed within a reactor vessel and a coolant, such as water or liquid sodium, is made to flow through the core so as to transfer the nuclear heat to the reactor coolant. The heated fluid is then used to generate steam which is used to drive conventional steam turbine-electrical generator apparatus.
Control of the reactor is usually achieved by control rods which are dispersed throughout the nuclear core and are mounted for movement into and out of the core. The control rods function by absorbing excess neutrons produced by the nuclear reaction; proper radial distribution of the control rods produces a substantially uniform power distribution across the core. On the other hand, proper axial positioning of the control rods permits the nuclear reactor to achieve design power levels.
Given the above nuclear, thermal, fluidic flow, and mechanical functions, and the attendant constraints and requirements associated therewith, it is readily understandable that a complicated and sophisticated structure for supporting the core, a sufficiently rigid coolant flow channeling means, and a precise control rod guide means are necessary within the reactor vessel. Moreover, since most typical commercial nuclear power plants are required to be refueled on the order of once every year, the complete apparatus within the reactor vessel is required to be disassembled in order to allow replacement of the nuclear fuel elements in the core. Since the reactor must be positively shut down during the refueling operations to comply with reactor safety standards, the control rods in the control rod guide means are often designed so as to be capable of being left in the core throughout reactor refueling. This, of course, even further complicates the design of the apparatus within the reactor vessel. In the prior art, the requirement of leaving the control rods in the nuclear core during refueling is achieved by providing a manually operated disconnect joint between the control rod and the control rod drive shaft. However, the disconnect joints are such that the reactor vessel must be unsealed to allow installation of a special tool which is used to manually actuate the disconnect joint, thereby uncoupling the control rod from the drive shaft. Manual actuation of the disconnect joint is also required to recouple the control rod through the drive shaft. While this prior art type of joint provides for high reliability, there are a number of disadvantages inherently associated with it. For example, a relatively large amount of time is required to wait for the reactor to cool down, both thermally and nuclearly, before the reactor vessel may be unsealed and personnel can safely pass; a relatively long amount of time is necessary for manually disconnecting each of the joints; and the long time required to manually disconnect the joints increases the radiation exposure of personnel working on the refueling.
To overcome the aforementioned problems, control rods are being designed which provide a disconnect coupling between a control rod and its drive shaft which have a high degree of reliability while allowing for remote coupling and uncoupling of the control rods from the drive shaft by utilizing the actuating force of the control rod drive mechanism. In utilizing this type of control rod, the precise relationship between the control rod and the drive shaft must be known at all times. The operator of the nuclear reactor system must know whether the control rod is coupled to the drive shaft, or whether the control rod is disengaged from the drive shaft. As an example of when this knowledge is necessary, during refueling operations, when it is desired to remove the drive shaft from the interior of the reactor vessel while maintaining the control rod within the core, the operator must know whether the drive shaft is disengaged, or uncoupled, from the control rods prior to removing the drive shaft from the reactor vessel.